How to (quantitatively) use the Smallest Green Patch to Achieve the best Cooling Effect in Urban Climate Adaptive Planning?

Utilization of urban green space (UGS) has been recognized as a promising option to mitigate urban heat island (UHI) effect. Previous studies have proved that the UGS has a significant negative correlation with urban temperature, and can effectively reduce the ambient temperature and form a "cool island effect (UCI)." However, how to (quantitatively) use the smallest UGS to achieve the best cooling effect is still unclear. Therefore, this paper proposed and defined the cooling extent, cooling intensity, cooling efficiency, and the threshold value of efficiency (TVoE). With the selection of 15 cities around the world and the employment of remote sensing, Google Cloud Computing, and Python-based statistics analysis, we found that the cooling effect of UGS has a TVoE, and the Normalized Difference Vegetation Index (NDVI), background temperature, and the climate model to which the city belongs have significant effects on the cooling effect of UGS, as well as the TVoE. Specifically, we found that (1) increasing (artificial) rainfall and irrigation contribute to improving the cooling intensity of grassland in Temperate Monsoon Climate (TMC) and Mediterranean Climate (MC) cities, particularly in the hot-dry environment. (3) The cities with high relative humidity would restrict the cooling effect of UGS; and increasing wind speed would significantly enhance the tree-covered while weakening the grass-covered UGSs’ cooling effect in MC cities. (4) We also identified that, in order to achieve the most effective cooling with the smallest sized tree-covered UGS, the area of trees in both (TMC and MC) climate zones’ cities should generally be planned around 0.5 ha. (5) Further, a two-tier (two optimal patch sizes:0.6 ha and 0.95 ha) distribution of the TVoE of tree-covered UGS in seven low latitude (hot-humid) Asian cities was found, which extends the understanding of TVoE in a regional (climate zone) scale. These findings extend the understanding of the UCI effect and TVoE of UGS as well as providing bases for scientific climate adaption planning globally.